How to make hollow object with only surface into solid?
It seems to me that you didn't finish the outline before you spun the model, so now you have three options. See the blender documentation for the Spin function.


You can modify your outline to have a top and bottom and re-spin it.
Select Edge Ring and press F to make a face.

2.1 Press Alt+P to Poke the face, thus making a triangle fan.
You can use Grid Fill or from the Ctrl+F menu.